Liquid metal monitors

ABSTRACT

A liquid metal monitor comprising an orifice in a liquid metal flow path which orifice can be at least partially plugged by impurity precipitated from liquid metal in the flow path, a division of the liquid metal flow at the orifice into two parts so that subsequently one of the parts passes through and the other part by-passes the orifice, characterized in that heat transfer means are provided for transferring heat energy from the liquid metal in the flow path upstream of the orifice to liquid metal in the flow path down stream of the orifice.

United States Patent Roach et al. [451 June 27, 1972 [54] LIQUID METAL MONITORS 2,997,874 8/1961 Billuris et a]. ..73/6I LM 2,782,369 2/1957 Werner et a1... ...73/6l LM UX [721 Invemm Francis Rolchs Wamnston; Daniel 3,002,820 10/1961 Hall et al. ...73/61 LM ux g Davids, Bowdon, both of 3,390,571 7/1968 Roach et a1. ..73/6l LM g an [73] Assignee: United Kingdom Atomic Energy Authority, 'f' Prince London England Assistant ExammerJoseph W. Roskos Att0rr1eyLarson, Taylor & Hinds [22] Filed: Sept. 24, 1970 211 App1.No.: 75,157 [57] ABSTRACT A liquid metal monitor comprising an orifice in a liquid metal flow path which orifice can be at least partially plugged by im- [30] Foreign Applicaflun Priority Data purity precipitated from liquid metal in the flow path, a divi- Oct. 7, 1969 Great Britain ..49,317/69 sion of the liquid metal flow at the orifice into two parts so that subsequently one of the parts passes through and the other [52] U.S. cl. ..73/6l LM p y-p h orifice, h riz in h h tr n fer [51] lnt.Cl ,G01n 11/00, G01n 25/02 means are provided for transferring heat energy from the [58] Field of Search ..73/61 LM liquid metal in the flow path upstream of the Orifice to liquid metal in the flow path down stream of the orifice. [56] Rem-ewes Cited 4 Claims, 2 Drawing Figures UNITED STATES PATENTS 3,462,997 8/1969 Roach et al. ..73/6l LM LIQUID METAL MONITORS BACKGROUND OF THE INVENTION This invention relates to liquid metal monitors and is a modification of the liquid metal monitor disclosed in U.S. Pat. No. 3,390,571.

That patent discloses a liquid metal monitor of the kind comprising an orifice in a liquid metal flow path wherein the temperature of the liquid metal at the orifice is controlled so as to maintain a partial plug of impurity precipitate in the orifice, thereby restricting the flow through the orifice to a selected fraction of the unplugged flow. The temperature of the liquid metal at the orifice, when the amount of partial plugging is stable, is indicative of the impurity content of the liquid metal. According to the invention disclosed in U.S. Pat. No. 3,390,571 the monitor is characterized in that means are provided at the orifice for dividing the liquid metal flow into two parts, so that one of the parts passes through and the other part bypasses the orifice. Hereinafter such a monitor will be referred to as a monitor of the kind described.

SUMMARY OF THE INVENTION According to the present invention a liquid metal monitor of the kind described includes heat transfer means whereby heat energy istransferred from the liquid metal upstream of the orifice to liquid metal downstream of the orifice. In a preferred embodiment of the invention the heat transfer means comprises cooling means whereby liquid metal upstream of the orifice can be cooled, and heating means whereby liquid metal downstream of the orifice can be reheated, said cooling means and heating means effecting heat transfer between liquid metal in the monitor and heat exchange medium in a separate fluid circuit, said separate circuit including a cooler and a pump for the medium, and control means whereby the rate of pumping of the pump in the separate circuit is rendered partly dependent upon the rate of flow of liquid metal through the orifice and partly dependent upon the temperature of liquid metal in the region of the orifice thereby creating a double servo loop control which serves to reduce disturbances of the temperature reading due to change in liquid metal inlet temperature. The preferred embodiment of liquid metal monitor finds application in monitoring liquid metal having radioactive contamination.

DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which FIG. I is a flow circuit diagram, and

FIG. 2 is a part cut away perspective view of components linked as indicated in FIG. 1 and incorporated in a nuclear reactor installation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In both Figures the same reference numeral is used for a given component though all components are not necessarily shown in both Figures.

Broadly FIG. I shows a liquid metal monitor having three interlinked main sections namely a liquid metal circuit section I, a heat transfer medium section II, and a pumping control section III.

I LIQUID METAL CIRCUIT SECTION A channel 11 carries a supply of liquid metal sodium to be monitored. Such sodium is drawn from the channel 11 by means of electro-magnetic pump 12 which serves to drive the sodium through a duct 13. Downstream of pump 12 the duct is surrounded by a first jacket 14 through which the heat exchange medium of the invention (in this case embodied by a mixture of liquid metal sodium and potassium) can be passed as will be described hereafter.

An orifice plate 15 is positioned in duct 13 downstream of the first jacket and acts to divide sodium flow along duct 13 so that it either passes into a bypass duct 16 or through the orifices in plate 15 into an orifice duct 17. A thermocouple 23 is embedded in the duct wall in the region of plate 15.

Downstream of orifice plate 15 the orifice duct 17 is surrounded by a second jacket 18 which is coupled to the first jacket by pipe 18a and incorporated in the heat exchange medium circuit as will be hereinafter described.

Bypass duct 16 opens into a return line 19 leading back to the channel 11. The bypass duct contains a valve 20 and a magnetic brake 21 governing the fraction of total flow in duct 13 passing through the bypass duct.

Orifice duct 17, also opening into return line 19, includes an electromagnetic flow meter 22 whose output (a function of the sodium flow through orifice duct 17 is fed to the pumping control section as will be hereinafter described.

II HEAT TRANSFER MEDIUM SECTION This section is made up of a closed loop 30 for coolant consisting of a liquid metal sodium potassium mixture which is driven around the loop by liquid metal pump 31 whose speed of operation can be varied. The liquid metal mixture in the loop is cooled by means of an air cooler 33 having vanes over which air is driven by a fan (not shown). An electro-magnetic brake 32 serves to limit convection flow in the coolant liquid metal mixture which flow can otherwise persist in the loop with the pump 31 switched off.

III PUMPING CONTROL SECTION This section on the basis of the outputs of thermocouple 23 and electromagnetic flowmeter 22 governs the operation of pump 31. The thermocouple and flowmeter outputs are fed into a differential DC amplifier 35. The output of the amplifier 35 is proportional to the differential of the two inputs and the varying output is used to effect correspondingly variable control of the pump 31. The saturation temperature represented by the output of thermocouple 23 is continuously recorded on a pen recorder 34. The output of flowmeter 22 is fed to the differential amplifier 35 by way of a controller 36.

FIG. 2 shows the circuit of FIG. 1 incorporated in an installation for the primary coolant circuit of the Prototype Fast Reactor at Dounreay. Items corresponding to those of FIG. I are given the same reference number though not all items of FIG. 1 are shown in FIG. 2.

The installation comprises a removable stainless steel pod 50 in which the circuit items are enclosed. The pod 50 is mounted inside a fixed outer vessel 51 which depends from a thick concrete biological shield 52. The removable pod 50 is located on the shield 52 by bolts 54 extending through flange 55 incorporated in the pod 50. The bolts engage flange 56 embedded in the shield 52. In the pod there is shielding 57 which is co-extensive with the shield 52.

Under normal operating conditions the outer vessel 51 contains liquid sodium metal (serving as the coolant for the reactor fuel elements) up to the level indicated by arrow 58. Sodium metal is circulated by external means (not shown) through inlet 59 into the outer vessel and returned to the bulk of sodium through outlet 50. Sampling ports 61 enable sodium within the outer vessel 51 to be drawn into, and rejected from the pod 50. The sampling ports 6! correspond to the pipes dipping into the channel 1 l of FIG. 1.

It will be noted that items in that part of pod 50 beneath shielding 57 are those of the liquid metal circuit section I of FIG. I (typically pump 12, brake 21, flowmeter 22, orifice plate 15 and jackets 14, 18). Likewise items in that part of the pod above shielding 57 are those items of the heat transfer medium section II other than the jackets 14, 18. These jackets are linked to the pump 31 and air cooler 33 in the upper part of the pod by lines 62, 63 which form part of closed loop 30 of FIG. 1.

Spindle 65 extends downwardly through shielding 57 to enable control of valve 20 (partly obscured) referred to in connection with FIG. 1.

By mounting the monitor in a pod in the way described the monitor can be removed for maintenance since radiation makes maintenance or repair in situ" in the outer vessel 51 impracticable. It is considered inexpedient in the present embodiment to blow cooling air directly on to pipes carrying radioactive sodium as was disclosed in US. Pat. No. 3,390,571. The intermediate sodium potassium mixture heat exchange circuit is here used to overcome this inexpediency. The principles of the monitor were discussed in the above Patent and are not detailed further.

Sodium metal from channel 1 l is driven through duct 13 by means of pump 12. Cooling of the sodium metal in duct 13 occurs by heat exchange with liquid sodium potassium mixture driven through thefirst jacket 14. Cooled sodium in duct 13 then arrives at the orifice plate 15 and divides into two parts one part passing into bypass duct 16 and the other part passing through the orifices of plate 15 into the orifice duct 17. Both orifice and bypass flows receive heat in passing through the part of duct 17 surrounded by second jacket 18. This jacket receives by way of pipe 180 the hot sodium potassium mixture from first jacket 14 which served to receive heat from liquid sodium upstream of orifice plate 15.

Liquid sodium in orifice duct 17 is then returned to channel 11 by way of return line 19 after passing through the electromagnetic flowmeter 22. Flow. through the duct 16 is governed by valve 20 and magnetic brake 21 before being restored to channel 11 by way ofretum line 19.

In US. Pat. No. 3,390,571 the cooling of the flow of liquid metal being monitored is related only to the output of the flowmeter in the duct through which liquid metal passes after it has traversed the orifice. A thermocouple at the orifice region served only as a temperature sensor. In the present embodiment the output of flow-meter 22 is fed by way of controller 36 into differential amplifier 35 together with the output of thermocouple 23. In this way the rate of cooling provided by the sodium potassium mixture in its closed loop 30 in passing through jackets 14, 18 is made a function of both flow through the orifice plate and the temperature of liquid metal in the region of the orifice plate.

When the temperature of the supply sodium remains constant, automatic control of the instrument operates around the loop comprising orifice flowmeters 21, controller 36, differential amplifier 35 and coolant pump 31. Their control system is similar to that described in U.S. Pat. No. 3,390,571. The effect of the orifice thermocouple connection is to introduce a degree of negative feedback.

When the temperature of the supply sodium varies, the temperature of the orifice (which is displayed as the saturation temperature) will also vary, although the actual impurity saturation temperature may have remained constant. Changes in the orifice temperature are communicated to the differential amplifier 35 which in turn controls the pump 31. The pump (and hence the coolant flow and orifice temperature) is adjusted in such a way as to reduce the the original orifice temperature excursion. Thus, the error in the saturation temperature reading caused by a change in inlet sodium temperature, is reduced by the application of feedback.

Controller 36 embodies an adjustable "set-point the position of which determines the value of the restricted orifice flow. It also embodies variable terms for optimizing controlloop performance.

In a second embodiment of the invention the construction is generally similar to that described in US. Pat. No. 3,390,571, with the exception that the electrical heater downstream of the orifice is replaced by an air heater. The air heater is fed with hot air leaving the cooler upstream of the orifice and transfers the heat to the liquid metal downstream of the orifice, subsequently discharging the cooled air to the atmosphere. Such a construction of liquid metal monitor is suitable where the liquid metal is not actively contaminated.

We claim: 1. In a liquid metal monitor of the kind comprising an on fice in a liquid metal flow path which orifice can be at least partially plugged by impurity precipitated from liquid metal in the flow path, and a division of the liquid metal flow at the orifice into two parts so that subsequently one of the parts passes through the orifice and the other part bypasses the same, the improvement comprising heat transfer means for transferring heat energy from the liquid metal in the flowpath upstream of the orifice to liquid metal in both parts of the flowpath downstream of the orifice, said heat transfer means comprising cooling means for the liquid metal in the flowpath upstream of the orifice and heating means for reheating the liquid metal in both parts of the flowpath downstream of the orifice, a separate fluid circuit between which and the liquid metal of the monitor heat transfer is effected by said cooling means and heating means, said separate fluid circuit including a heater, a cooler, a pump for the heat exchange fluid medium, and control means for rendering the rate of pumping of the heat exchange fluid medium in said separate circuit partly dependent upon the rate of flow of liquid metal through the orifice and partly dependent upon the temperature of the liquid metal in the flowpath in the region of the orifice thereby creating a double servo loop which serves for reducing disturbances of the temperature reading, which reading serves for determining the impurity content of the liquid metal, due to changes in the liquid metal inlet temperature.

2. A liquid metal monitor according to Claim 1, wherein the heat exchange medium in the separate circuit is a liquid metal sodium-potassium mixture which is driven round the circuit by a liquid metal pump and is cooled by means of an air cooler.

3. A liquid metal monitor according to claim 2, wherein the heater in the separate circuit is an electrical heater.

4. A liquid metal monitor according to claim 2, wherein the heater in the separate circuit is an air heater fed with hot air leaving the air cooler.

* t F i i 

1. In a liquid metal monitor of the kind comprising an orifice in a liquid metal flow path which orifice can be at least partially plugged by impurity precipitated from liquid metal in the flow path, and a division of the liquid metal flow at the orifice into two parts so that subsequently one of the parts passes through the orifice and the other part bypasses the same, the improvement comprising heat transfer means for transferring heat energy from the liquid metal in the flowpath upstream of the orifice to liquid metal in both parts of the flowpath downstream of the orifice, said heat transfer means comprising cooling means for the liquid metal in the flowpath upstream of the orifice and heating means for reheating the liquid metal in both parts of the flowpath downstream of the orifice, a separate fluid circuit between which and the liquid metal of the monitor heat transfer is effected by said cooling means and heating means, said separate fluid circuit including a heater, a cooler, a pump for the heat exchange fluid medium, and control means for rendering the rate of pumping of the heat exchange fluid medium in said separate circuit partly dependent upon the rate of flow of liquid metal through the orifice and partly dependent upon the temperature of the liquid metal in the flowpath in the region of the orifice thereby creating a double servo loop which serves for reducing disturbances of the temperature reading, which reading serves for determining the impurity content of the liquid metal, due to changes in the liquid metal inlet temperature.
 2. A liquid metal monitor according to Claim 1, wherein the heat exchange medium in the separate circuit is a liquid metal sodium-potassium mixture which is driven round the circuit by a liquid metal pump and is cooled by means of an air cooler.
 3. A liquid metal monitor according to claim 2, wherein the heater in the separate circuit is an electrical heater.
 4. A liquid metal monitor according to claim 2, wherein the heater in the separate circuit is an air heater fed with hot air leaving the air cooler. 